This invention generally relates to methods and apparatus for processing semiconductor wafers, in particular to rapid thermal processing methods and apparatus for processing semiconductor wafers.
Rapid thermal processing (RTP) has become widely used in various stages of semiconductor manufacturing. For example, rapid thermal processing is used for the chemical deposition of various films on semiconductor wafers. Rapid thermal processing has also been used in the annealing of ion or dopant implanted semiconductor wafers. Because the operating temperature in rapid thermal processing can be rapidly increased or decreased, the required processing time is short and the efficiency is high.
In many RTP applications, the heat treatment often needs to be conducted under specific atmospheric conditions with a specific gas composition flowing through the processing chamber where the wafer is being treated. FIG. 1 illustrates an example of a conventional prior art RTP reactor for heat-treating a semiconductor wafer in a flowing gas composition. The reactor 10 has a processing chamber 11 where a semiconductor wafer can be treated. A rotatable susceptor 12 is mounted within the processing chamber 11. A wafer 13 to be processed is held and supported by susceptor 12, and can be rotated along with the rotatable susceptor 12. In addition, the apparatus also has a gas delivery system including a gas inlet 14 at one end of the processing chamber 11, and a gas outlet 15 at the other end of the processing chamber 11. A gas composition typically flows into processing chamber 11 at room temperature, while the operating temperature in processing chamber 11 is very high, e.g., from about 500xc2x0 C. to about 1200xc2x0 C. A heat source 16 is located above processing chamber 11 such that the surface of wafer 13 can be radiated by the heat emitted from heat source 16. Heat source 16 comprises a plurality of heating units each having therein a halogen lamp 17.
As is well known in the art, it is critical that during the rapid thermal processing of a wafer, the entire surface of the wafer is heated uniforrnly. Non-uniformities in temperature distribution across the wafer surface can result in dislocations and distortion in the wafer. However, in conventional rapid thermal processing, it is often very difficult to achieve temperature uniformity across the wafer surface. As is apparent from FIG. 1, it is difficult to design the spatial arrangement of the wafer and the individual heating units so that the radiant heat energy received at each point on the wafer surface is the same. As a result, radial temperature gradients from the edge to the center of the wafer can be formed. In particular, the outer edge of the wafer often receives the least radiation energy. In addition, rapid thermal processing is typically conducted in a very short cycle, e.g., 2-15 minutes. The wafer is rapidly heated to a very high temperature and is cooled rapidly. Any small variations in heat radiation at different points of the wafer surface can cause drastic temperature variations. As a result, the thermal stress at different points of the wafer surface can vary, causing the distortion of the wafer. In the case of rapid thermal chemical vapor deposition, the deposition rate at the different points on the wafer surface can vary because of the temperature non-uniformity, thus resulting in non-uniformity in the thickness of the deposited film.
There has been a great deal of effort in the art in developing satisfactory solutions to the problem of heat non-uniformity. Many different methods have been proposed, and yet the heat non-uniformity problem remains.
U.S. Pat. No. 4,789,771 discloses a number of ways to compensate for thermal non-uniformity. For example, the patent proposes coating the processing chamber walls with a medium to reflect light from the heat lamps, energizing the outermost heat lamps to produce higher temperatures than the centrally located lamps, and varying the spacing of the heat lamps to compensate for thermal non-uniformity.
U.S. Pat. No. 4,533,820 describes a heating apparatus for radiant heating semiconductor wafers in a flowing gas. The apparatus has a processing chamber where a wafer is treated. The gas flows into and out of the processing chamber through a gas inlet and a gas outlet. A gas flow dispersion barrier having a plurality of gas through-holes is disposed within the container. The dispersion barrier is said to improve the gas flow uniformity, thus improving the heat uniformity on the wafer surface.
Much effort has been focused on the real-time control of the radiation energy emitted from individual lamps to achieve heat uniformity. Typically such control systems include measuring temperatures at different points of the wafer surface and adjusting the radiation energy output of different heating units based on the measured temperatures.
In one method, an optical pyrometer is used with a wavelength of two to three micrometer (xcexcm), to monitor temperatures at different points on the wafer surface. The monitored temperature parameters are then processed by a processor such as a computer to generate a new set of parameters. The power of the individual heating units is then adjusted based on the new parameters to compensate for the temperature differences at different points on the wafer surface to achieve temperature uniformity. However, in this method, in order to achieve temperature uniformity, it is important that accurate temperature measurements are obtained by the pyrometer. In practice, the temperature measurements of the pyrometer often are distorted and inaccurate because of the interference by various factors in the processing. Examples of such factors include the reflectivity of the wafer, the radiation from the radiant lamps, the radiant energy which has passed through the wafer or around it, the radiant energy reflected back by the walls of the processing chamber, and the heat energy generated in the chemical reaction during the deposition process, etc.
To obtain more accurate temperature readings, U.S. Pat. No. 5,154,512 discloses the so-called xe2x80x9cripple technique.xe2x80x9d The method utilizes a first optical fiber to measure the infrared light emission from the wafer in a narrow band, and a second optical fiber to measure the light emission from the lamps in the same narrow band infrared region. The radiation measured by the first optical fiber is used to determine the light reflected from the wafer. The variation in the measured light reflection is used to deduce the reflection coefficient for the wafer at a particular wavelength. The coefficient is then used to calculate the temperature on the wafer surface based on the radiation collected by the first optical fiber.
U.S. Pat. No. 5,841,110 discloses another control method, which includes measuring the broad band reflectivity of the wafer to be treated before the wafer is placed in the RTP processing chamber. The measured reflectivity is then used by the RTP system to adjust the RTP system parameters used in the rapid thermal processing of the wafer.
Despite the effort in the art, non-uniformity in temperature distribution on wafer surface in rapid thermal processing remains to be a problem. Thus, there is still need in the art for improved rapid thermal processing methods which can achieve better heat uniformity and less distortion in the processed wafer.
This invention provides a method for rapid thermal processing a semiconductor wafer under atmospheric conditions, e.g., in a flowing gas composition. Improved temperature uniformity on the surface of the wafer being treated can be achieved by the method, resulting in significantly less dislocation or distortion in the processed wafer.
In accordance with the present invention, the gas composition is preheated before it is flushed into the rapid thermal processing chamber. Typically, the gas composition is heated to a preheat temperature that is sufficiently close to the operating temperature of the thermal processing chamber such that when the gas composition reaches the wafer being treated, its temperature is substantially same as the operating temperature. Preferably, the gas composition is preheated to the operating temperature before it flows into the processing chamber. Because the difference between the entering gas temperature and the operating temperature in a processing chamber is drastically reduced or even eliminated, when the gas composition flows into the processing chamber, it will not absorb any substantial amount of heat from the outer edge of the wafer being processed. Thus, the interference with the temperature uniformity on the wafer surface by the entering gas composition is minimized. Improved temperature uniformity can be achieved on the wafer surface.
In another aspect of this invention, a rapid thermal processing apparatus is provided. The apparatus has a preheat unit for preheating a gas composition, and a reactor for the rapid thermal processing of a semiconductor wafer. Typically, the reactor has a processing chamber and a radiant heat source for heating the wafer in the chamber.
In a preferred embodiment, the preheat unit has a preheat chamber, and a preheat source and preheat temperature sensor each being operably coupled to the preheat chamber, a preheat controller operably coupled to the preheat source, and a processor operably coupled to both the preheat controller and the preheat temperature sensor. When a gas composition is preheated, the preheat temperature sensor monitors the temperature of the gas composition, and the preheat controller controls the preheat source. The processor receives signals from the preheat temperature sensor and the preheat controller, processing the signals, and feeding control information to the preheat controller such that the preheat controller adjusts in real time the power of the preheat source in response to the temperature monitored by the preheat temperature sensor.
In another embodiment of the invention, the rapid thermal processing reactor has a wafer temperature monitor for measuring a temperature on a surface of the wafer, and a heat source regulator operably coupled to the radiant heat source for controlling the power of the radiant heat source. The processor receives signals from the preheat temperature sensor, the wafer temperature monitor, the preheat controller and the heat regular, processes the signals, and outputs control information to the preheat controller and the heat source regulator. The preheat controller and the heat source regulator then adjust the power of the preheat source and the radiant heat source respectively according to the control information. As a result, the gas composition is preheated in response to the measured wafer temperature, and the wafer is heated uniformly.
As discussed above, in conventional RTP methods, a gas composition of room temperature is supplied directly into the RTP processing chamber. Because of the drastic difference between the temperature of the gas composition and the operating temperature in the processing chamber, the gas composition can significantly interfere with heat uniformity on the surface of a wafer being treated by absorbing heat from at least the outer edge of the wafer. By preheating the gas composition, the present invention significantly reduces the temperature difference, and improves the temperature uniformity on the wafer surface. As a result, less dislocation and distortion in the processed wafer is caused, and wafers with better qualities can be produced.